os: fix tmpdir on unix

vlib/os/os.v

@@ -1073,27 +1073,6 @@ pub fn join(base string, dirs ...string) string {
 // tmpdir returns the path to a folder, that is suitable for storing temporary files
 pub fn tmpdir() string {
 	mut path := os.getenv('TMPDIR')
-	$if linux {
-		if path == '' {
-			path = '/tmp'
-		}
-	}
-	$if freebsd {
-		if path == '' {
-			path = '/tmp'
-		}
-	}
-	$if macos {
-		/*
-		if path == '' {
-			// TODO untested
-			path = C.NSTemporaryDirectory()
-		}
-        */
-		if path == '' {
-			path = '/tmp'
-		}
-	}
 	$if windows {
 		if path == '' {
 			// TODO see Qt's implementation?
@@ -1108,6 +1087,9 @@ pub fn tmpdir() string {
 			}
 		}
 	}
+	if path == '' {
+		path = '/tmp'
+	}
 	return path
 }